Qt6 migration: Fix BUILD_RTI=OFF build errors in TrainingAreaBase

- training_area_telegrams.cpp: Fixed QJsonObject initializer syntax (Qt6 requires {{}} not ({}))
- training_area_telegrams.cpp: Fixed fromJson() methods to access m_* members directly
- training_area_telegrams.cpp: Fixed brace mismatches in multiple toJson() methods
- training_area_telegrams.h: Changed Header struct to public members for consistency
- tx_mixin.cpp: Fixed getEquipmentNumber() template for direct member access
- lts_coordinate.cpp: Added missing #include <iomanip>
- MIGRATION_STATUS.md: Added migration progress tracking document

C++17 constraint enforced due to RTI Connext DDS 7.7.0 incompatibility with C++20+

Build status: BUILD_RTI=OFF successful (0 errors, warnings only)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

qt6-after/qt6-migrated-legatus/Lts/Viewer/SituationMap/Map/map_initializer.cpp

@@ -20,12 +20,12 @@
 
 #include <arcgis/arcgisversion.h>  // This file is created by RME, not ESRI
 
-#include <QCoreApplication>
-#include <QJsonDocument>
-#include <QJsonObject>
-#include <QJsonArray>
+#include <QtCore/QCoreApplication>
+#include <QtCore/QJsonDocument>
+#include <QtCore/QJsonObject>
+#include <QtCore/QJsonArray>
 #include <QNetworkProxy>
-#include <QMessageBox>
+#include <QtWidgets/QMessageBox>
 
 
 //----------------------------------------------------------------------------